In the typical fluid catalytic cracking unit (FCC unit), a particulate catalyst is cycled between a cracking zone in which the hydrocarbon feedstream is cracked and a regeneration zone in which the coke deposited on the catalyst is burned off in the presence of oxygen. Since the coke generally contains some sulfur compounds (the amount usually depends on the sulfur content of the hydrocarbon feed), during regeneration significant amounts of sulfur dioxide and sulfur trioxide are formed. These sulfur oxides formed in the regenerator may be controlled by including in the circulating inventory of the unit a sulfur oxide sorbent, such as reactive alumina, capable of sorbing the sulfur oxides in the regenerator and releasing them as hydrogen sulfide in the presence of hydrocarbons in the cracking vessel.
The efficiency of such a sulfur oxides control system is enhanced by the presence of a sulfur dioxide oxidation promoter, such as platinum. These sufur dioxide oxidation promoters aid in the formation of sulfur trioxide in the regenerator which is more readily sorbed by the sulfur oxide sorbent than sulfur dioxide. However, a disadvantage of this method of controlling sulfur oxides is that the promoter has been observed to increase the amount of nitrogen oxides present in the regenerator flue gas. Since the oxides of nitrogen are themselves noxious gases, it is desirable to control the amount of nitrogen oxides, especially nitric oxide (NO), in the flue gas.